The invention relates to a tool for setting self-piercing rivets, in particular solid rivets or semitubular rivets, having a hold-down for prestressing the parts to be joined, in particular sheets, a riveting punch which is guided so as to be axially movable in a cylinder of the hold-down and can be acted upon by a force, and a die which is opposite the hold-down and has a prominence on a section of its end face facing the riveting punch.
The setting of rivets is described in detail, for example, in the publication xe2x80x9cNietsysteme, Verbindungen mit Zukunftxe2x80x9d [Riveting systems, joints with a future] (cf. U. Klemens and O. Hahn: Nietsysteme, Verbindungen mit Zukunft [Riveting systems, joints with a future] publishing association: Interessensgemeinschaft Umformtechnisches Fxc3xcgen und Laboratorium fxc3xcr Werkstoff-und Fxc3xcgetechnik der Universitxc3xa4t-GM Paderborn.-Sonderausgabe-Holzminden: Hinrichsen, 1994, pages 18 to 20). During self-piercing riveting with a solid rivet, complete filling of the annular groove of the rivet by the die-side sheet is an essential condition for high transmittable forces. With the known tools, however, this filling of the annular groove of the rivet is not always completely successful This is due to the fact that, when a solid rivet is being set in the die-side sheet, locally undesirable deformations may already occur at the start of the riveting operation. The deformations occur at the outer margin of the circular-ring-shaped prominence of the end face of the die. In the further course of the embossing operation of the riveting process, the displaced material which has flowed into the deformations is then unavailable for filling the annular groove of the solid rivet. The result is that the riveted connection produced in this way does not achieve the strength which it should have.
During self-piercing riveting with semitubular rivets, an important parameter to achieve is a considerable spread of the rivet shank, this parameter having a substantial effect on the forces which can be transmitted by the connection. The upsetting of the semitubular rivet is intended to achieve gap-free positive locking of the parts to be joined. However, practical experience with the use of the known tools, and in particular when riveting sheets which are relatively hard compared with the hardness of the semitubular rivet, has also shown that the rivet foot is not sufficiently spread and is then upset to a pronounced degree. A proper undercut, which is actually decisive for the strength of the connection, is not achieved.
The object of the present invention, which arises from these shortcomings with the use of known tools for setting self-piercing rivets, and in particular also when riveting sheets which are relatively hard compared with the hardness of self-piercing rivets, is to develop the known tools in such a way that the abovedescribed defects in the finished riveted connection do not occur. At the same time, the development of the known tools is to be simple, uncomplicated and inexpensive.
To achieve the object, provision is made for the end face of the die to be split into sections outside the prominence, and for the individual sections of the end face to be made so as to be mutually displaceable in the axial direction as a function of the penetration depth of the respective self-piercing rivet into the parts to be joined or of the force which the riveting punch exerts on the self-piercing rivet. In this way, when a solid rivet is being used, for example, the generation of an undesirable local deformation in the die-side sheet at the start of the riveting operation is effectively prevented. In this phase, those sections of the end face of the die which are located outside is then upset to a pronounced degree. A proper undercut, which is actually decisive for the strength of the connection, is not achieved.
In Document D1, according to DE-A-44 19 065, a tool for setting tubular rivets, for example, has been disclosed, having a hold-down, for prestressing sheets of different thickness, a riveting punch which is guided so as to be axially movable in a cylinder of the hold-down and can be acted upon by a force, and a die which is opposite the hold-down and has a prominence on a section of its end face facing the riveting punch, the end face of the die being split into sections radially outside the prominence, namely into an inner mandrel section having a prominence and an outer annular die section, and the individual sections of the end face are mutually displaceable in the axial direction as a function of the penetration depth of the tubular rivet into the sheets or of the force which the riveting punch exerts on the tubular rivet. This teaching for the setting of tubular rivets cannot be readily applied to the setting of solid rivets, for it is necessary in the case of solid rivets as self-piercing rivets to completely fill an annular groove in the shank of the solid rivet with the material of the parts to be joined in order to obtain a satisfactory riveted joint. its annular prominence bear firmly against the die-side sheet and prevent the generation of a deformation. During the further progress of the riveting operation, these surface sections then give way in the axial direction relative to the annular prominence. The annular prominence retains its original position. That material of the die-side sheet which is otherwise xe2x80x9clostxe2x80x9d due to the undesirable deformation is therefore retained and, as intended, can flow into the annular groove of the solid rivet and completely fill the latter.
The procedure is different when processing semitubular rivets. Here, the hump-shaped prominence in the center of the end face of the die projects relative to the surrounding surface sections at the start of the riveting operation. The result of this is that the parts of the material to be joined which are located under the recess of the semitubular rivet are first of all caused to be bent into the recess of the semitubular rivet in a curved manner. With the progress of the riveting operation, the hump-shaped central prominence of the die then gives way in the axial direction, so that a riveted connection can be produced in which all the components associated with one another are connected in the predetermined manner. In particular, a closing head of regular form is obtained, with which the semitubular rivet is sufficiently spread without excessive upsetting of its cutting margins and where the sheets to be riveted to one another have flowed into the recess of the semitubular rivet to a uniform thickness.
A further advantage of the development according to the invention consists in the fact that the especially high rigidity, required according to the prior art, of the joining device is no longer necessary to the same extent. Since the improved riveting tools produce a continuous flow and joining of the parts to be connected to one another, especially high rigidity of the joining devices can largely be dispensed with, which enables the tools to be used in a more universal manner.
In the simplest case, the force under which the sections of the end face which surround the prominence of the end face are axially displaceable relative to the prominence is produced by the force of a preloaded spring as a function of the penetration depth of the self-piercing rivet into the sheets to be joined. This involves a passive control of the axial movement of the surface sections of the die. Such a control is preferably applied if a simple and continuously increasing force/displacement characteristic for the joining process produces a good result. In this case, simple control can be realized with mechanical elements, e.g. disk, helical or plastic springs. The position and shape of the characteristic are predetermined here within the tool by the rigidity and preloading of the springs.
It is far more efficient, however, if this force is set as a function of the pressure of the riveting punch. In this case, the axial displacement of the end face can at the same time also be set as a function of the hardness of the materials to be riveted. In the simplest case, a hydraulic unit which is known per se and has an adjustable pressure relief valve is used as the pressure-medium source.
In particular, the tool according to the invention is configured in such a way that, for setting a solid rivet, the sections of the end face of the die which concentrically surround the annular prominence of this end face are designed so as to be axially displaceable relative to the annular prominence. In the other case, when setting a semitubular rivet, where a hump-shaped prominence is arranged in the center of the end face of the die, this end face is configured in such a way that that surface section of the end face which has the hump-shaped prominence is axially displaceable relative to the outer sections of the end face of the die which surround it concentrically.
By means of a hydraulic unit, variable force/displacement characteristics can be set externally, i.e. they can be freely programmed. In the case of programmable controls, electrohydraulic servo valves known per se are used for setting the characteristic. The strength of the riveted connection, in particular the dynamic strength, can be substantially improved with a variable characteristic, and in some applications can also be achieved only in this way.
When setting solid rivets, the cutting process can be influenced in a positive manner with regard to a neat cut surface in the punched hole and with regard to the prestressing state after the riveting. These important parameters depend on increasing wear of the cutting edge of the die. The wear of this cutting edge is compensated for by increasing the axial height of the annular prominence of the cutting edge of the die as wear progresses. This adjustment may be effected continuously and externally. In the simplest case, the volume of the hydraulic pressure medium is reduced if the outer die part is designed as a piston.
When setting semitubular rivets, the sequence of motion is set in such a way that a large counterforce is preset at the start of the riveting operation. This promotes the spreading of the semitubular rivet at the start of the riveting operation. During the further course, a counterforce which can be decreased continuously is produced in order to obtain an especially good undercut in this way.